Copy keyframes ending on current frame
Not sure if this is what you are asking, but you can try this. Switch to the Dope Sheet so you can see all your keyframes. Select the one you want to duplicate, hit SHIFT+D and the drag the mouse to where you want it to go. You can also SHIFT+select to duplicate multiple keyframes.

In your case, just move move the duplicated keyframe to the end of the timeline.

NOTE: I think the best option for you would be to type in a number instead of dragging your mouse. EG: typing "10" will move the keyframe 10 frames ahead. Subtract the target frame (the one you want to move the duplicated keyframe to) by the start frame (the frame you are duplicating it from).